Page 153 - Materials Chemistry, Second Edition

136 Practical Design Calculations for Groundwater and Soil Remediation

Solution:
(a) Insert the known values into Equation (4.24) to find out the value
of τ:

50
C out − (0.05)τ
= = e
C in 1200
τ = 64 min
(b) The required reactor volume, V = (τ)Q (from Equation 4.24)
= (64 min)(30 gal/min) = 1,920 gal

Discussion:
1. To achieve the same conversion, the size of the PFR, 1,920 gallons
(this example), is much smaller than 13,800 gallons for the CFSTR
(Example 4.16).
2. The design equations for batch reactors and PFRs are essentially
the same. The required reaction times for these two reactors
are the same, at 64 min. The actual tankage of the PFR is much
smaller because loading and unloading time does not need to be
included in operation of flow reactors.

4.6 Reactor Configurations
In practical engineering applications, it is more common to have a few
smaller reactors than to have one large reactor for the following reasons:

• Flexibility (to handle fluctuations of flow rate)
• Maintenance consideration
• A higher removal efficiency

Common reactor configurations include arrangement of reactors in series, in
parallel, or a combination of both.

4.6.1 Reactors in Series
For reactors in series, the flow rates to all the reactors are the same and equal
to the influent flow rate to the first reactor, Q (Figure 4.2). The first reactor,
with a volume V , will reduce the influent COC concentration, C , and yields
0
1
an effluent concentration, C . The effluent concentration from the first reactor
1
becomes the influent concentration to the second reactor. Consequently, the